My son's high school allows them, and it surprised me at first. But one of the reasons they used in explaining is that kids no longer wear watches, but use their phones to check the time. Each teacher gets to set the rules for what is allowed in their own classroom (turned completely off, set to vibrate, even just left on the teacher's desk, etc.), so the teacher's authority is not undermined there. The students have to follow each teacher's guidelines.
I have been very relieved that I have a way to get in touch with him, even when it is not an emergency (and face it, there are actually relatively few real emergencies).
Another matter that the school mentioned is that, whether we like it or not, cell phones are a part of current culture and will be until something else comes along. All kinds of changes have come about in schools because of new inventions and new technology, through the years.